Baseball caps and other forms of headwear are popular all over the world for people of all ages. To accommodate variations in head circumference and comfort, a single style of headwear must come in a number of different sizes or be adjustable. In an effort to minimize the number of sizes offered, certain types of headwear are equipped with an adjustment mechanism whereby a person wearing the hat can adjust the circumference of the hat based on his or her head size. The current invention relates generally to adjustment mechanisms for headwear, and to headwear having such adjustment mechanisms.